6.3 Anaerobic Metabolism: Glycolysis
The Glycolysis Pathway (Anaerobic)
Glycolysis is another metabolic pathway that generates ATP anaerobically by using carbohydrate in either the form of glucose or glycogen.
Glycolysis begins with the six-carbon, ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. In the first part of the glycolysis pathway (A), two ATPs are required so that the six-carbon sugar molecule can be split evenly into two three-carbon pyruvate molecules. In the second part of glycolysis (B), four ATPs and two NADHs are produced. After subtracting the two ATP required upfront, the net yield of glycolysis when starting with glucose is 2 ATP, 2 NADH, and two pyruvate molecules. Muscle glycogen enters glycolysis partway through the first phase of glycolysis, and only one ATP is required upfront. This means that when starting with muscle glycogen, the net yield of glycolysis is 3 ATP, 2 NADH, and two pyruvate. This is why exercising muscles preferentially use muscle glycogen instead of blood glucose when glycogen is available.
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Image Description
- Two Pyruvate: Two pyruvates move to the next stage of cellular respiration.
- Glucose Molecule: The 6-carbon glucose molecule is split into 2 separate 3-carbon pyruvate molecules.
- Net gain of 2 ATP: Produces 4 ATP but uses 2 ATP, resulting in a net gain of 2 ATP released into the cell for energy use.
- 2 NADH: Produces 2 NADH that travel to the mitochondria, carrying high-energy electrons to the ETC.
Glycolysis Summary
- Yields enough ATP for approximately 1 to 3 minutes of intense activity.
- Relies on the supply of glucose, which can be present in the blood or as glycogen in muscle.
- Pathway creates a by-product called lactic acid.
- Pathway requires anywhere from several minutes to 2 hours of rest to fully recover, depending on how much lactic acid is present.
Fitness Fact
Lactate or lactic acid is a byproduct of glycolysis when the demand for glucose outweighs the oxygen available to help break it down. It is a common myth that the accumulation of lactate in the muscle contributes to the ‘burning’ or sore feeling many of us experience during or shortly after we exercise. While lactic acid does build up during strenuous activity, it’s the dissociation of lactate that leads to acidity of muscle tissue through the accumulation of positively charged hydrogen ions (H+).
“8.4 Glycolysis” from Nutrition and Physical Fitness by Angela Harter Alger is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
